Battery Capacity of Mercedes EV is 67% Bigger than for BMW i3

The two newest electric cars on the market—both luxury models from Germany—make for a fascinating study of contrasts. The BMW i3 and Mercedes B-Class Electric Drive carry nearly identical price tags of $42,300 and $42,400 respectively. They are both beautifully designed and well engineered. But it’s their differing approaches to overcoming limitations in driving range—arguably the biggest limiting factor for battery-powered cars—that is most instructive for the future of EVs.

The official EPA range estimates hide these essential differences. The window sticker indicates a driving range for the BMW i3 at 81 miles, while the Mercedes B-Class’s driving range this week was pegged by the EPA at 85 miles. That’s a negligible difference. Unfortunately, the estimates are wrong.

Efficiency Is Good

It’s well known that EPA efficiency and range numbers are consistently misleading. For electric cars, I look past those numbers and instead laser-focus on the size of the battery pack. I use this rule-of-thumb: every kilowatt-hour of capacity provides between three and four miles of range.

Great aerodynamics and lightweight materials—and a gentle touch with the accelerator pedal—can mean four miles or more per kilowatt-hour. Drive like a demon in a heavier blunter car, and efficiency drops closer to 3 miles/kWh.

The size of the BMW i3’s battery pack is commonly called out as 22 kilowatt-hours. Actually, we should focus on the “usable” portion of the pack—because engineers don’t allow the full potential of energy storage to be depleted. (Running a battery pack down to zero reduces the longevity of the battery.) The usable capacity of the i3’s pack is 18.8 kWhs.

Tom Moloughney, who contributes to this site and puts a lot of miles on his i3, tells me that he gets about 80 miles per charge in his car. (That’s purely on electricity, and for the sake of simplicity, let’s leave aside the issue of range-extending gas engines.) Eighty miles from the i3's pack represents phenomenal efficiency. In fact, it breaks my rule-of-thumb because it means the i3 is capable of closer to 4.5 miles per kilowatt-hour in the pure EV version of the car, which goes further than 80 miles purely on electricity.

This extraordinary level of efficiency—and the use of a relatively smaller pack—is only possible because BMW built the i3 from the ground-up as a dedicated EV with superior aerodynamics and lightweight carbon fiber materials.

But Size Counts

Mercedes, on the other hand, used an existing platform for the B-Class Electric Drive. Nonetheless, it’s small and efficient—although not groundbreaking. However, the Mercedes EV has a significantly larger battery pack: It’s 64 percent bigger!

Michael Schweizer, program manager for e-mobility at Mercedes-Benz USA, informed me by email yesterday that the B-Class Electric Drive’s total battery capacity is 36 kilowatt-hours. The number usually ascribed to the B-Class is 28 kWhs—and that’s what the EPA used for its 85-mile driving range estimate.

Yes, under “normal” charging, the Mercedes EV fully charges to 28 kilowatt-hours. But when the “range plus package” is employed, it pushes utilization to about 31.4 kilowatt-hours. Using that number compared to the i3’s usable capacity of 18.8, and the delta between the two is actually 67 percent.

Those 31.4 kilowatt-hours, with a mid-range level of efficiency of 3.5 miles/kWh delivers a genuine 100-mile EV—to be exact, 110 miles. Compare that with the superlative efficiency of 4.5 miles per kWh on the i3. Its18.8-kWh pack yields an impressive 85 miles of range.

Yet, all things being equal (including final price of the car), it looks like a bigger battery strategy wins for total range. I know that all things are not equal, just as I know that EV drivers love a good argument. In fact, the idea for this post was inspired by a discussion I had this week with Martin Eberhard, founder of Tesla, who said that nobody except Tesla is building EVs with big enough packs. Okay, let the arguments begin.

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Comments

Horses for courses. BMW and Daimler did indeed take very different approaches. I applaud BMW's holistic approach, something they plan to build on with the new i brand as an entire line of highly efficient BEVs and PHEVs will follow the i3 in coming years. It's clear they have lofty long term goals and the i3 is as much a learning experience with regards to working with carbon fiber and other new materials as it is a car to them.

Daimler's approach of mating a Tesla powertrain to an existing vehicle platform is nearly the complete opposite. However they didn't have to hack apart a car that was never meant to be electric, like a RAV4 EV or a Fiat 500e, Daimler designed the new B-Class platform from the start to accept both the Tesla powertrain and their conventionally fueled offerings.

Still, as you mention, having a battery that is 64% larger does allow for more range, even with a vehicle that is much heavier and less efficient. However it only manages 7% more range (81mpc compared to 87mpc) in standard charging mode. That's because Daimler holds a much larger percent of the battery in reserve. (22% compared to the i3's 13% reserve). I say "standard charging mode" because like Tesla products, the B-Class has an optional "range package" ($600) that allows the owner to press a button before charging which will open up an additional 3 to 4 kWh's of battery that is usually held in reserve and that will add approximately 15 miles of extra range.

This is a very nice option, but Daimler cautions not to use it frequently or you will suffer from early battery degradation. I've asked how "How much is too much?" but haven't been able to get a really clear answer on that yet. The best I could get was they don't recommend you using it more than a couple times a month. Using the range mode should make the B-Class a "real" 100-mile EV.

I do like the B-Class and am weighing it against getting a second i3. They both have advantages and disadvantages and the pricing is nearly identical, although I can get a fully loaded BEV i3 for about $1,500 less. I'm tempted to go for the B-Class just so I could do some interesting side by side comparison tests, but the i3 is faster and offers a more sporty driving experience. Once I see some inventory at my local Mercedes dealers I'll likely make up my mind and add the second EV to my garage. Two cars and a house powered by my solar array; I've looked forward to this for a long time. So happy it's finally happening!

@Tom M: Many folks would likely be appreciative if you go for the B-Class rather than another i3 so that you can make comparisons based on long term ownership. If I were cross-shopping those vehicles, I'd definitely go for the B-Class with the "range package". I partially agree with Martin Eberhard; only Tesla, in 2014, is building EV battery packs with "enough" range (that includes the RAV4 EV and B-Class packs). The LEAF's ~80 miles were fine for a new car in 2011, but I don't think it's unreasonable to expect that new EVs today should generally be able to achieve a true 100 miles of range.

It seems like neither one quite hit the mark. The larger from the Mercedes should have been put in the nice light-weight BMW i3. Then they would have had a nice pure EV with a nice healthy range. Perhaps BMW will eventually offer a larger battery . . . I'd rather have that than the $4K 2-banger with a tiny gas tank.

On Wednesday, I entered into the world of EV's with my new lease of the Mercedes B Class ED. After 2 days with the car I love it. It has great acceleration and I like being able to control the amount of regenerative braking with the paddle shifters. I ordered this car before anyone knew much about the "range package" and unfortunately I did not choose that option (mainly because my daily commute is 6 miles) and I rarely take long trips in my car. In hindsight, it is worth the $600 whether you would use it or not. I'm hoping that maybe the "range function" can be added at a later time with a software update.

The only objektive way to compare ev's is to compare tests done by qualified organisations. Subjective opinions about the range is whortless. I for one have regular 100 miles range on my i3 driving to work and back pluss some extra. I can not use this to claim 100 miles range for the i3. The same apply for other cars like the B class in this case. The numbers are clear, the extra 6 kw battery only give 6 miles of range. Accept it.
The explanation of why can be discussed, but the test can not.

I think i need to clearify my previous post a bit, it was done on one of these handheld devices that are not easy to write a lot of text on.
The main issue i have with this article the suggestion is that EPA is not usable to compare range between EV's, and that it is better to use rules of tumbs or subjective observation of range acomplished by someone. This is like replacing sience with religion. The EPA consist of a wide variaty of test situations that together will give the combine range you can expect, taking cold, hils and highway driving into consideration. The EPA range is not the maximum range of the car, and it is not the minimum, but it reflects that particular cars efficiancy the best way possible. So the BMW i3 actually have a lot better MPG than the B-class. It has a range between 80 to 110 miles. I do not know what the B class have, but i know the EPA of both.
the conclution for me is that the B class is a bad design when it needs 28 kw of battery to accomplish 6 miles more range than the I3. Of cause the B class can will have more range than the i3 when using of the reserve capasity, but it is still a poor result.

I never would've considered a Mercedes before this article. As soon as I saw the similar price and longer EV range I had to look at their site.
One really nice option is a heated front windshield to reduce energy use.
That's a very interesting upgrade, I would opt for that in a heartbeat.
But sadly, it's still a short range EV.
I'll keep my Volt till something else comes along. Model3, Volt2, etc…

I have been pleasantly surprised at how not-inconvenient a 75-mile range EV is. Only on rare occasions is range an issue and now that I know where most of the (50+) charging stations in Las Vegas are and I have my ChargePoint account setup and tested, it's even easier.